Drop on demand ink jet technology for producing printed media has been employed in commercial products such as printers, plotters, and facsimile machines. Generally, an ink jet image is formed by selective placement on a receiver surface of ink drops emitted by a plurality of drop generators implemented in a printhead or a printhead assembly. For example, the printhead assembly and the receiver surface are caused to move relative to each other, and drop generators are controlled to emit drops at appropriate times, for example, by an appropriate controller. The receiver surface can be a transfer surface or a print medium such as paper. In the case of a transfer surface, the image printed thereon is subsequently transferred to an output print medium such as paper. Some ink jet printheads employ melted solid ink.
The image is typically made up of a grid-like pattern of potential drop locations, commonly referred to as pixels. Variations in color may be achieved by selectively depositing ink drops at the potential drop locations by using dithering or halftoning techniques. Dithering, or halftone printing, uses an aggregation of monochromatic dots to produce different shades of gray or other colors. Halftone reproductions rely on the ability of the human eye to integrate a plurality of small black dots on a white background and perceive the dot covered area as a shade of gray. Thus, white areas typically have 0% coverage, and solid color areas have 100% coverage. The percentage coverage, or fill, of an arbitrarily selected unit area may be used to identify the gray level of the unit area. For example, a unit area having one-half of its area covered by ink drops may be defined as having 50% coverage, or 50% fill.
Ink jet printers can produce undesirable image defects in the printed image. One such image defect is non-uniform print density, such as “banding” and “streaking.” “Banding” and “streaking” are caused by variabilities in volumes of the ink droplets ejected from different ink drop generators. Such variabilities in ink volume may be caused by variability in the physical characteristics (e.g., the nozzle diameter, the channel width or length, etc.) or the electrical characteristics (e.g., thermal or mechanical activation power, etc.) of the drop generators. These variabilities are often introduced during print head manufacture and assembly.
Methods of reducing banding artifacts caused by nozzle-to-nozzle differences are known. For instance, in some prior art systems drop volume variability between nozzles of a printhead has been reduced by “normalizing” each jet or nozzle within a printhead. Normalization of the printhead nozzles is accomplished by modifying the electrical signals, or driving signals, that are used to activate the individual nozzles so that all of the nozzles of the printhead generate an ink drop having substantially the same drop mass. Normalization of the jets in the printhead may be effective in the generation of substantially uniform drop mass across the nozzles of an individual printhead. In multiple printhead systems, however, the “normalized” drop mass produced may vary from printhead to printhead resulting head-to-head banding defects which may cause noticeable color variations and/or hue shifts and generate images that do not accurately replicate desired colors.
Methods have been developed to address drop volume variation between printheads. For example, U.S. Pat. No. 6,154,227 to Lund teaches a method of adjusting the number of micro-drops printed in response to a drop volume parameter stored in programmable memory on the print head cartridge. Also, U.S. Pat. Nos. 6,450,608 and 6,315,383 to Sarmast et al., teach methods of detecting inkjet nozzle trajectory errors and drop volume using a two-dimensional array of individual detectors. These methods, however, require the use of sophisticated sensors and ink cartridges. The calibration time, cost, and physical space constraints may weigh against the use of these and other possible complex methods.
Another method comprises detecting the average drop mass output by each printhead at a single fill level, such as 100% fill, for example. The average drop mass output by a printhead may then be adjusted to be within specification by uniformly increasing or decreasing the voltage level of the driving signals that activate the drop generators of the printhead. Testing has shown, however, that small head-to-head drop mass variations may be visible throughout dithered fill patterns. Testing has also shown that the average drop mass may vary considerably from head-to-head when printing halftone fill patterns even after drop volume between printheads has been normalized at 100% fill. For example, FIG. 1 is a graph showing drop mass deviation at 25% fill and 100% fill for a printhead assembly that has already had 100% fill drop mass set to within specification. Notice that, although the drop mass variation at 100% is within +/−0.5 ng, the head-to-head drop mass variation at 25% fill is greater than +/−1.5 ng. Specifications may require drop mass variations to be as low as 0.5 ng. Thus, while the 100% fill head-to-head drop mass variation is within specification, the head-to-head drop mass variation at 25% is not. Consequently, the head-to-head drop mass variation at 25% fill may be noticeable to printer operators as head-to-head banding.